This invention generally relates to ultrasound imaging systems. In particular, the invention relates to methods and apparatus for imaging moving fluid and tissue.
Ultrasonic imaging systems, as commonly used for medical imaging, require an array of ultrasonic transducers contained within a probe to provide steering and focusing of the ultrasonic beam. Each transducer transmits a high-frequency signal into the object being examined and receives a high-frequency echo signal returned from the object following transmission. In particular, respective transmit pulses are transmitted to the transducers of the probe from a transmit beamformer incorporated in the host electronic system or image console. The echo signals detected by the transducers of the probe must then be transmitted to the host electronic system or image console for receive beamforming and signal processing. Even with advanced very-large-scale integration microelectronics techniques, only a relatively small part of the overall system electronic signal processing may reside in the probe.
The maximum number of simultaneously active elements in a transducer array of a medical diagnostic ultrasound probe has steadily grown over the past 15 years. This trend continues and may actually accelerate, due to the use of large two-dimensional apertures in three-dimensional image reconstruction systems. If a separate coaxial cable is used to convey the detected echo signal obtained from each of a large number of transducer array elements of the ultrasound probe to the main signal processing unit, then the bundle of cables attached to the probe can become stiff, making manipulation of the probe for imaging difficult. Probes with roughly 500 cables are commercially available, but they are regarded as relatively clumsy. On the other hand, probes with 128 cables are routinely used by sonographers without complaint.
This problem has been recognized for some time and some innovations have been proposed to address it. One solution was proposed in U.S. Pat. No. 5,566,133, which discloses a method whereby the coaxial cables are replaced by fiber-optic cables. Since fiber-optic cables are more flexible, a larger number can be used while retaining ease of maneuverability of the probe. However, in order to transmit signals with the required dynamic range, this method requires the use of digital optical modulation, which in turn requires that time-gain compensation and analog-to-digital conversion functions be incorporated in the probe for each array element. This involves a considerable amount of power to operate and can be relatively expensive to manufacture. Thus there is a need for a technique whereby the detected echo signals for the multiplicity of transducers in the probe can be communicated to the host signal processing electronics without need to incorporate one coaxial cable for each transducer.
Communication between a large number of ultrasonic transducers incorporated in a probe and a central electronic unit or computer of an ultrasound imaging system by means of coaxial cables can require fewer cables than the number of transducers by using frequency division multiplexing to enable a single coaxial cable to carry the signals from multiple ultrasound transducers in an array. The invention enables a bundle of, for example, 128 coaxial cables to be used to transport the signals originating from an array of, for example, 1024 active transducers or elements. In this example, the ratio of active array elements to coaxial cables is 8:1. Such a ratio requires frequency division multiplexing of eight separate array elements onto a single coaxial cable, which requires that the single coaxial strand have relatively high bandwidth. For example, to multiplex element output data from a 5-MHZ probe at an 8-to-1 ratio would require in excess of 80 MHz (two-sided) bandwidth on the individual coaxial cable. This might require that the gage of the individual strand be increased, and a good trade-off may have to be found between multiplex ratio, cable gage and flexibility of the bundle of coaxial cables.
In accordance with the preferred embodiments of the invention, different modulation schemes for individual signals can be utilized. The final choice between these methods will depend on the available frequency response characteristics of the individual coaxial cables. In accordance with one preferred embodiment, if the cable bandwidth is limited, single-sideband, suppressed-carrier amplitude modulation can be used to pack the element output signals into the available bandwidth. This would require filtering the mixer output signal to reject image spectra prior to signal addition. In accordance with another preferred embodiment, if cable bandwidth is large (or if high-bandwidth cables could be obtained to replace those currently in use), then double-sideband, suppressed-carrier amplitude modulation could be used. Although this would be wasteful of bandwidth, it would allow the element output data to be communicated from the probe to the central electronic processing unit or computer without filtering for rejection of frequency-domain images. It would be advantageous to implement the modulation without filtering, but, again, this depends on cable bandwidth.